Field of the Invention
The present invention relates to a protective film material that is coated on a surface of a wafer to provide protection when the wafer is irradiated with a laser beam to carry out a processing operation such as scribing, dicing or grooving. The present invention also relates to a wafer processing method using the protective film material.
Background Information
Circuit devices, such as IC, LSI and LED devices, are formed on a surface of a wafer made of a compound such as silicon or sapphire. The surface of the wafer is partitioned by division lines, called streets, provided lengthwise and widthwise on the wafer surface. Respective regions of the wafer surface partitioned by the streets define the circuit devices which are separated into individual circuit devices by cutting the wafer along the streets. Prior to the separation process, the wafer is subjected to laser processing during which the streets are irradiated with a laser beam to form processing grooves, after which the circuit device are separated along the grooves.
When the wafer is subjected to laser processing as described above, wafer forming components (e.g., atoms) vaporized (sublimed) due to laser exposure scatter, and diffuse and fall on the surface of the wafer as foreign matter (debris) and adhere to the wafer surface, including the circuit devices. However, the debris adhered to the surface of the wafer cannot be removed by a subsequent process, such as a water rinse process. As a result, the debris can impact the yield and reliability of the circuit devices formed on the wafer.
As a countermeasure to overcome the foregoing drawbacks of the prior art, in order to control the surroundings during laser processing to avoid the wafer from being influenced by the debris resulting from the scattering of the wafer vaporized (sublimed) components, processing methods such as spraying of gas, vacuum suction, or processing under reduced pressure have conventionally been utilized. However, since such conventional processing methods require the installation and operation of apparatuses and general working conditions which are complex and time consuming, further improvement has been demanded.
Under such circumstances, to solve the foregoing problem caused by debris, a method has been employed in which a protective film comprising a resin is formed on a surface of a wafer to be processed, and the wafer surface is irradiated with a laser beam through the protective film. By this processing method, even if the debris is scattered, the wafer surface can be protected by the protective film and the debris does not attach to the circuit devices.
As the resin forming the protective film, JP-A-2007-073670 proposes the use of polyvinyl alcohol (PVA) that is coated on the wafer surface for protection using a spin coater, for example. After laser processing is carried out, the debris on the PVA coating is washed away simultaneously with the washing of the protective film with water.
While PVA is conveniently used because it can be easily removed by washing with water as described above, however, it is still impossible to prevent the deposition of debris completely. Even after the removal of PVA by washing after the laser processing, a part of the debris attaches to the wafer surface, particularly in the portion adjacent to processed grooves, and cannot be completely removed. Further, as described below, the wafer surface may sometimes have stain-like residues which are considered attributable to thermal change of PVA due to attachment of debris under heated conditions. As a result of the foregoing problems, sufficient protection of the wafer surface cannot be achieved.